This invention relates to continuous fiber preforms for reinforced composites and composites containing such preforms. More particularly, this invention relates to carbon fiber preforms comprising continuous tows of carbon fiber disposed as chords of a circle to form a near net shape part.
The present invention is particularly concerned with improved carbon-carbon fiber composites intended for use in applications where severe shear stresses will be encountered, for example, by being subjected to circumferential stress. A prime example of such use is a friction disc employed in a disc brake. Such discs are essentially annular in shape, having at least one surface of each disc being provided with a friction-bearing surface. Braking is accomplished through contact between the friction-bearing surfaces of the discs, thereby converting the mechanical energy of the rotating brake to heat. In addition to withstanding the shearing stresses, the discs also are required to act as heat sinks, absorbing high heat loads. Because of its strength, density, heat capacity, thermal conductivity, coefficient of friction and stability to its sublimation temperature (about 3600xc2x0 C.), carbon has been particularly attractive for use in constructing such disc brakes, particularly where weight is a major consideration such as in aircraft. Strength and stiffness of a composite are controlled by the orientation of the reinforcement fibers in the matrix. When reinforcement fibers are straight and continuous, the stresses are efficiently transferred through the composite in the direction of the fiber. If the reinforcement fibers are crimped or discontinuous, efficiency drops as the stresses are transferred out of the reinforcement and through the matrix.
In the prior art, composites have generally been fabricated by orienting or directionally aligning the carbon fiber component, which generally has been thought necessary in order to take advantage of fiber strength and enhance mechanical properties of the composite. Fabricating the composite with the desired fiber orientation is more readily accomplished by use of continuous carbon fiber, and such fiber has been preferred over discontinuous fiber for these applications. The primary forms of continuous fiber employed in composite fabrication include woven textile fabric or unidirectional tapes for use in lay-up structures, and continuous fiber yarn or tow, which are used for filament winding of hollow cylindrical shapes and in braided structures.
For example, in U.S. Pat. No. 4,790,052 to Orly carbon composite brakes are produced using a quasi-isotropic layup of uni-directional webs as the reinforcement. These webs are layered so that angles of around 0xc2x0/xe2x88x9260xc2x0/+60xc2x0 are formed between the filaments of successive layers of the structure. Though the total web stack is quasi-isotropic, each web is an extremely unbalanced reinforcement which must withstand multi-dimensional forces being transferred through a uni-directionally reinforced web. At load points such as the lugs of a brake disc, very high interlaminar stresses are generated and may cause failure through delamination. To resist these stresses, a very high degree of needlefelting is used to add reinforcement through the stack which will crimp the reinforcement fibers and reduce their effectiveness. The Orly method has economic and performance drawbacks as well. The method is complex and leads to poor material utilization because unidirectional webs which are rectangular are used to make annular shaped parts.
An improvement in the Orly method was made in U.S. Pat. No. 5,184,387 to Lawton. In Lawton, a unidirectional layer of filaments is subjected to needlefelting to provide dimensional stability. The layer is then cut into a plurality of arc-shaped segments and those segments are assembled side by side to produce the annular shape. This methodxe2x80x94similar to that used in the garment industryxe2x80x94reduces the wastage of material and permits the layer to be cut so that the filaments run radially in some segments and circumferentially in others. This method yields a 0/90xc2x0 layup in that the filaments in a superimposed layer of segments is disposed at a 90xc2x0 angle to those filaments of a lower layer of segments. The resulting composite is less isotropic than Orly and also requires a high degree of needlefelting to prevent delaminations.
Still another method is disclosed in U.S. Pat. No. 5,217,770 to Morris et. al. This method uses an annular braid web to form a mat which is then needlefelted to form a 3-D structure. The braid contains helical fibers woven at approximately 40 degrees and unidirectional fibers which become circumferential during laydown. One or more braids are used to fill the brake annulus which is then needlefelted. This method achieves a near net-shaped part with fibers oriented to handle circumferential and shear loads. However, using braids to build the structure is a complex textile operation that adds to the cost. In addition, the helical fiber volume and angle is not uniform throughout the annulus due to the use of a tubular braid to form an annulus. When a straight braid is curved into an annulus the fibers in the braid are forced closer together at the inner periphery of the annulus and pulled apart at the out periphery of the annulus changing their angles and fiber density.
As noted, needlefelting is widely used in the textile arts to strengthen stacked fabric structures and improve structural integrity. Generally described, needlepunching operations are carried out by forcing barbed needles normally through the stack layers in the thickness direction. A portion of the fiber within the fabric layers is gathered by the barbs and repositioned in the thickness direction, reinforcing the individual fabric layers as well as the stack. If the fibers making up the layers are continuous, the needlepunching operation necessarily breaks individual filaments when re-orienting them. To avoid or at least minimize such breakage, improved processes wherein staple fiber is included within the structure, either as part of the fabric layer or as alternating layers of staple fiber sheet, have been used to supply staple fiber to the needles for re-orienting in the needlepunching operation. Needlepunching operations have been employed in the art with carbon fiber sheet and tape to provide preform structures having good integrity for use in the production of carbon-carbon fiber reinforced composites.
The high degree of fiber alignment within the structure of these prior art composites is intended to take advantage of the strength and dimensional stability of the carbon fiber. However, composites having the entire fiber content aligned in a single direction are highly anisotropic in character, exhibiting a high degree of strength and dimensional stability in the fiber direction while suffering greatly reduced strength properties and poor dimensional stability in the transverse direction. To ensure that the strength of the composite, as well as its heat transfer characteristics and other important properties, are reasonably uniform and to minimize unidirectional shrinkage which may cause warping and distortion, the fiber direction is varied throughout the prior art structures, imparting some isotropic character to the composite. The lamina, however, still suffers from anisotropic effects. When using more costly fabric or the like, the fabricator still has had to resort to varying fiber orientation between successive layers of the structure, for example, using a 0/90xc2x0 orientation in one layer, +/xe2x88x9245xc2x0 in the next, and so on, thereby providing a composite having less anisotropic characteristics at the lamina level and being nearly quasi-isotropic overall. As described above, three-dimensional weaving, needlepunching and similar operations are often employed to add through-thickness fiber orientation and improve interlayer strength properties to accommodate these anisotropic stresses. However, a preform with isotropic character throughout the structure, especially at the lamina level, in the fiber reinforcement continues to be difficult to attain.
Current methods for producing carbon-carbon fiber reinforced composites exhibit further shortcomings. For most applications, finished carbon parts generally are made to precise dimensions, and their production requires conducting extensive shaping and machining operations on carbonized or fully graphitized carbon-carbon fiber composite blanks. Precision machining operations are expensive to carry out and difficult, and great care is needed with carbon-carbon fiber composites to avoid cracking or other damage. Carbon blanks having substantially the finished shape and dimensions, termed net shape blanks, would reduce the extent of machining needed and significantly lower costs. However, carbonized preforms are generally friable and cannot be readily formed or shaped. Constructing shaped preforms from layered fabric or fiber sheet thus generally requires cutting component parts having the desired final shape from fabric sheet before stacking and needlepunching. Such cutting operations are wasteful and produce considerable quantities of scrap fabric. Even when suitable methods for recycling of the scrap are found, the production and re-processing of scrap further increases the energy and waste disposal burdens already imposed on the manufacturing process, significantly raising the overall cost of producing the carbon article.
Methods for forming non-woven webs of carbon fiber have also been disclosed in the art, for example in U.S. Pat. No. 4,032,607 to Schulz. According to patentees, particularly attractive webs may be formed from mesophase pitch by melt- or blow-spinning the pitch, air-layering or water-layering the resulting fiber either as-spun or after being chopped, and thermosetting or air-oxidizing the non-woven web to stabilize the structure before carbonizing. Generally, the resulting webs are composed of random filaments rather than filament bundles or tow, and take the form of low density, thin felts and papers with very low bulk densities, generally well below about 0.3 g/cc. Non-woven webs may be suitable for use in forming layered carbon-carbon fiber structures in the same manner as continuous fiber tape and fabric by employing prior art layering and needlepunching operations such as those described herein above. Even after the needlepunching, structures comprising such highly randomized filaments generally will have a low fiber volume and consequently a very low density. Such structures do not provide the strength advantages generally obtained when using dense, high fiber volume structures comprising aligned and oriented continuous fiber, either in woven textile form or as unidirectional fiber tape.
A method for fabricating carbon fiber preforms and carbon-carbon fiber composites having superior strength properties and good thermal characteristics from substantially continuous fiber, preferably in a near net-shape and avoiding the use of unidirectional tapes, fibers or webs that exhibit reduced strength and poor dimensional stability in the transverse direction, would be particularly valuable to the carbon composites art.
Thus, it is an object of the present invention to provide improved reinforced composites. It is a further object to provide high strength fiber reinforced structures for composite materials. It is a further object of the present invention to form a near net-shape circular or annular friction disc preform resulting in less wastage of costly reinforcement fiber. It is a further object of the invention to provide a preform with reinforcement fibers that are disposed as chords of a circle at numerous angles and that remain as straight and continuous as possible across the preform to maximize reinforcement effectiveness. It is still a further object of this invention to provide a friction disc of nominally isotropic properties in the plane with a wide range of fiber orientation on the lamina level and throughout the overall composite. Finally, it is an object of the invention to provide a method of making a friction disc preform that can be easily modified to change the tow size, number of tows used and the chordal angles selected for winding to optimize the desirable mechanical and wear properties of a carbon/carbon brake.
Chordal preforms having a three-dimensional fiber structure suitable for use as reinforcement in the manufacture of composites and particularly desirable for use in producing high-strength, high thermal conductivity carbon-carbon fiber reinforced composites may be produced by disposing fibers as chords of a circle to fill the circle, or an annulus within the circle, of a friction surface which is then needlefelted into a 3D felt. Disposing the reinforcement fibers as chords of a circle, for standard geometries, creates a preform or a portion of a preform that is substantially isotropic at the lamina level and which contains reinforcement fiber tows that are non-unidirectional and sufficiently straight to achieve reinforcement effectiveness. The needlefelting serves to increase the density of the structure and to re-orient a portion of the fiber in the thickness direction to improve integrity and strength characteristics. The preform may conveniently be produced directly from fiber as a net-shape preform having the general overall shape of the final product, together with dimensions adjusted to accommodate for such shrinkage as may occur during subsequent thermal treatment. The net-shape winding process minimizes the scrap production and concomitant waste encountered in prior art processes for fabricating high-strength reinforcing textile fabric, sheet and tape, and reduces the need for extensive machining and forming operations.
The preferred preform will generally comprise carbon fiber and, though made without resort to binders or the like, the mechanical strength of the preform is adequate to withstand subsequent carbon composite manufacturing operations including infiltration with pyrolytic carbon or impregnation with a carbonizable filler and subsequent carbonization. The preform may also have application in the manufacture of fiber-reinforced thermoset and thermoplastic resin matrices, metal matrix and ceramic matrix composite structures.
A dense carbon-carbon fiber composite may be readily produced by depositing pyrolytic carbon within the invented preforms using well-known chemical vapor deposition processes and infiltration operations generally known and widely employed in the composite art. Alternatively, the preform may be impregnated with a carbonizable filler, cured under pressure and heat, and then further heated to carbonize the filler together with any pitch fiber component present, thereby providing a dense carbon-carbon fiber composite. Multiple infiltrating or impregnating operations may be employed if needed to produce a product having the desired density, and the processes may be used in combination. As used herein, the term xe2x80x9ccarbonxe2x80x9d is intended to include both ungraphitized and graphitized carbon. Thus, carbon fiber preforms may comprise graphitized or partially graphitized carbon reinforcing fibers or a mixture thereof, and carbon-carbon fiber composites comprising such reinforcement embedded in a matrix of graphitized or partially graphitized carbon. Articles in which matrix, and possibly the fiber, are still in the thermoset state are also included.
Thus, in one embodiment this invention provides a preform wherein 5-100% of the preform fibers are disposed as chords of a circle. In another embodiment, this invention provides a composite preform comprising fibers disposed as chords of a circle to provide a preform that is substantially isotropic at the lamina level with respect to its strength, stiffness and thermal properties. In another embodiment, this invention provides a carbon fiber preform comprising carbon fiber tows disposed as chords of a circle to provide a preform that is substantially isotropic at the lamina level with respect to its strength, stiffness and thermal properties. In another embodiment, this invention provides a carbon fiber preform that is balanced and symmetric about any central axis or point comprising substantially continuous carbon fiber tows disposed as chords of a circle such that different chord angles are used at various planes of the preform so that properties needed at different planes of a wear disc are optimized. In another embodiment, this invention provides a carbon fiber preform comprising substantially continuous carbon fiber tows disposed as chords of a circle and chopped fiber disposed in the preform and preferentially aligned in the z-axis direction. Another embodiment provides a method of manufacturing a fiber preform comprising continuously disposing fibers as chords of a circle by winding the fibers around a mandrel at selected chord angles so that the preform has substantially uniform fiber areal weight with fibers disposed to incorporate a wide range orientations dispersed throughout the preform.